This invention relates to a WDM (wavelength division multiplexing) optical transmission device and a WDM transmission system. More specifically, the invention relates to a light emitting device using a laser element capable of efficiently taking out light beams as radiation mode via second-order Bragg diffraction gratings and to a photodetector device. The invention further relates to a WDM optical transmission system including these devices.
WDM optical transmission is currently remarked as a technology which promises a remarkable increase in capacity that can be transmitted through already-provided optical fibers. In WDM optical transmission, a plurality of distributed feedback laser diodes (DFB-LD) are used as its light source. These DFB lasers are capable of oscillating in single longitudinal modes in wavelength bands different from each other. In conventional WDM optical transmission, these DFB lasers were provided on the part of a transmitter in a parallel arrangement of individual elements, or in a higher advanced type as arrayed monolithic integrated elements.
FIG. 8A is a longitudinal cross-sectional view of a general construction of a conventional DFB laser. The DFB laser 110 is composed of a InGaAsP active layer 112 and an InGaAsP optical guiding layer 113 having a larger band gap than the active layer 112, which are stacked on an n-type InP substrate 111 as an active resonator. Further made thereon are diffraction gratings 117 including a .lambda./4 shift region 118 in the center of the cavity in which the phase is shifted by 1/4 wavelength. Stacked thereon is a p-InP layer 114 to form a p-n junction. AR (anti-reflection) coats 116 are applied on both facets.
FIG. 8B is a schematic diagram showing oscillation wavelength characteristics of the DFB laser 110. As shown here, the DFB laser oscillates in the wavelength band near the Bragg wavelength (.lambda..sub.Bragg). The "stop band" shown in FIG. 8B pertains to a state where transmitted light is minimized and most of light is reflected, as characteristics of the optical wave traveling through the waveguide.
The Bragg wavelength (.lambda..sub.Bragg) can be expressed as EQU N.lambda..sub.Bragg =2n.sub.eff .LAMBDA. (1)
where .LAMBDA. is the period of diffraction gratings 117, and n.sub.eff is the effective refractive index of the laser waveguide 113. N is the order of diffraction. When N=2, for example, namely, in case of second-order gratings, the period .LAMBDA. of the diffraction gratings is twice the case of the first order.
The .lambda./4 shift DFB laser, namely, the DFB laser having anti-reflection coats 116 on both facets to prevent reflection and having the .lambda./4 phase shift region 118 in the diffraction gratings 117 located at the center of the laser cavity, oscillates at the Bragg wavelength at the center of the stop band as shown in FIG. 8B.
On the part of a transmitter of WDM optical transmission, a plurality of .lambda./4 shift DFB lasers are arranged in parallel. They can be used as a WDM light source capable of oscillating at a plurality of wavelengths by changing periods of individual diffraction gratings.
FIG. 9A is a diagram schematically showing a basic structure of the WDM system. On the part of the transmitter, a plurality of DFB lasers 110 oscillating at different wavelengths .lambda..sub.1 to .lambda..sub.n in constant intervals are arranged. These DFB lasers 110 are modulated directly, and their optical outputs are multiplexed in the multiplexer 100 and transmitted through an optical fiber 200. On the part of a receiver, the optical signal is amplified by an optical amplifier 300, and demultiplexed into original wavelengths by a demultiplexer 400. Each demultiplexed light component is converted into an electric signal by an avalanche photodiode (APD), PIN photodiode (PIN-PD), or any other appropriate photodetector 20.
FIG. 9B is a diagram schematically showing another construction of the WDM system. In the construction shown here, lasers 110 are not modulated directly, but driven by a d.c. signal, and their optical outputs are modulated by external modulators 30. Therefore, the system shown here is more advantageous than the system shown in FIG. 8A in that the modulation speed is higher.
FIG. 10 is a perspective view schematically showing an exemplary construction of the transmitter of the WDM system shown in FIG. 9A. In the construction shown here, the transmitter includes an integrated DFB laser array 10A for five different wavelengths and a multiplexer made of a LiNbO.sub.3 -based Ti-diffused waveguide. Transmitters of this type are disclosed in, for example, Toshiba Review, 40[7](1985), Okuda et al. p.570, and its English version (1985) p.9, Optoelectronics Technology in Toshiba, H. Okuda et al.
In the laser array 110A shown in FIG. 10, DFB lasers 110 are arranged to align their active layer stripes in parallel, and are integrated monolithically. Each DEB laser emits its optical output from a facet at the end of the cavity stripe. That is, the laser array 110A emits laser beams corresponding to the number of channels in parallel from different facet positions. To collectively enter these optical outputs into a single fiber 200, the multiplexer 100 is indispensable. This means an increase in number of parts of the system and makes it complicated.
The use of the multiplexer 100 invites another problem that the multiplexer 100 must be assembled so that its portion for entry of light be in an accurate position relative to the optical output facet of the laser array 110A, and the alignment procedure of the optical axis takes time.
Additionally, the multiplexer 100 needs a long branch-shaped waveguide portion 100L, as shown in FIG. 10, in order to guide beams of the light from different entry positions into a single waveguide. Therefore, the multiplexer 100 needs a length longer than the laser array 110A, and results in occupying a large space and disturbing down sizing the transmitter.
A further problem with the multiplexer 100 lies in a high cost because elements obtained from a single wafer are much less when the multiplexer 100 is monolithically integrated at the output of the laser array 110A.
If the lasers are arranged in series to align their cavities coaxially for the purpose of omitting the multiplexer 100, then the light emitted from a former-stage laser adversely affect the cavity of a latter-stage laser to cause cross-talk of signals. Therefore, the multiplexer 100 could not be omitted from conventional WDM systems.
In the receiver of the WDM system, a demultiplexer 400 is indispensable to divide the multiplexed signal into different wavelengths to deliver them to individual photodetectors 20. Also the demultiplexer 400 involves the same problems, and has been desired to omit from the system from the viewpoint of the space and assemblage.